The present invention relates to printing ink compositions and methods using printing inks, especially lithographic printing inks. The compositions and methods of the invention employ novel vinyl resins that are prepared using polyfunctional monomers, particularly vinyl resins having low number average molecular weights and broad polydispersities.
Printing inks generally include one or more vehicles and one or more colorants as principal components. Printing ink vehicles must meet a number of performance requirements that include both requirements related to the printing process, such as suitable consistency and tack for sharp, clean images, suitable length to avoid fly or mist, or proper drying characteristics, and requirements related to the printed image, such as gloss, chemical resistance, durability, or color. In general, ink vehicles include one or more materials such as vegetable oils or fatty acids, resins, and polymers that contribute to the end product properties, and may include other components such as organic solvents, water, rheology modifiers, and so on that may affect body, tack, or drying characteristics.
Printing inks are employed in a variety of printing processes. Printing processes include letterpress printing, lithographic printing, flexographic printing, gravure and other intaglio processes, screen printing, and ink-jet digital printing. The ink composition is affected by the demands of the process used. For example, inks used in printing operations in which the ink will come into contact with rubber elements, such as the blanket of an offset lithographic press, generally include as the solvent portion of the vehicle only petroleum distillate fractions or other aliphatic solvents that will not adversely interact with rubber.
In lithographic printing, an inked printing plate contacts and transfers an inked image to a rubber blanket, and then the blanket contacts and transfers the image to the surface being printed. Lithographic plates are produced by treating the image areas of the plate with an oleophilic material and ensuring that the non-image areas are hydrophilic. In a typical lithographic printing process, the plate cylinder first comes in contact with dampening rollers that transfer an aqueous fountain solution to the hydrophilic non-image areas of the plate. The dampened plate then contacts an inking roller, accepting the ink only in the oleophilic image areas. The press operator must continually monitor the printing process to insure that the correct balance of the fountain solution and the ink is maintained so that the ink adheres to the printing areas, but only the printing areas, of the plate in order to produce a sharp, well-defined print.
The industry has long sought an offset printing process and associated materials that would not require a separate fountain solution. Waterless plates have been made by applying to the non-image area a silicone rubber, which has a very low surface energy and is not wetted by the ink. The silicone-modified plates are expensive, however, and require expensive, specially-cooled press equipment because the fountain solution of the traditional two-fluid method also serves as a coolant. Other efforts have been directed to producing a single-fluid lithographic ink, i.e., an ink that does not require a separate fountain solution, that can be used with the industry-standard presses and all-metal plates. Parkinson, in U.S. Pat. No. 4,045,232 (the entire disclosure of which is expressly incorporated herein by reference) describes lithographic printing and earlier efforts directed to producing a single-fluid lithographic ink and the tendency of single-fluid inks to be unstable. Parkinson notes that ink emulsions containing a solution of glycerin and salts tend to xe2x80x9cbreak,xe2x80x9d with the result that the glycerin wets the inking rollers preventing good inking. Parkinson suggests an improved single-fluid ink obtained by using an additive that includes a resin treated with a concentrated mineral acid, and, optionally, a polyhydric or monohydric alcohol. Preferred polyols are glycerin, ethylene glycol, and propylene glycol. DeSanto, Jr. et al, in U.S. Pat. No. 4,981,517 (the entire disclosure of which is expressly incorporated herein by reference) describe a printing ink that is an emulsion of an oil-based phase and a water-miscible phase. The patentees allege that an emulsion containing a significant portion of water (10% to 21%) and employing phosphoric acid as a critical component has improved stability against phase separation and can be used as a single-fluid lithographic ink. The De Santo, Jr. composition further includes as a diluent and emulsion stabilizer an oil with the properties of No. 1 and No. 2 fuel oils and a polyol emulsifier, of which glycerin and ethylene glycol are the only examples provided.
Nonetheless, due to various drawbacks of the single-fluid lithographic inks that have previously been proposed, including the limited stability and poor definition and toning already mentioned, the industry standard continues to be a dual-fluid lithographic ink that includes an ink component and a separate fountain solution component.
Applicants have now discovered that particular compositions that include a polyol phase dispersed or emulsified in a vinyl resin vehicle phase overcomes these problems in a single-fluid lithographic ink.
Yet another problem observed in printing processes, especially with high speed presses such as web-offset lithographic presses, is what has been termed xe2x80x9cink misting.xe2x80x9d Ink misting commonly refers to ink droplets that become airborne during the printing process, for example at the point where the rotating inked rollers separate. When two inked rollers are in contact, as they are for the ink transfer in offset printing, ink splitting can result in the formation of ink filaments that may break to produce ink droplets. When the droplets become larger or more like threads, the problem may be termed xe2x80x9cink slinging.xe2x80x9d The high speed of the presses and the modification of ink properties to print on such high speed presses exacerbate the problem of misting or slinging. While the mist can be annoying and cause contamination at low levels, higher amounts of misting can potentially create environmental and/or safety problems. The relative amounts of ink misting for particular inks can be determined by comparing the inks on an inkometer, employing standard testing procedures.
Various methods have been suggested for reducing misting or slinging in inks. For example, different additives have been proposed, including kaolin, anionic or cationic surfactants, or an additive that is an organic acid phosphate, glycerol, or propylene carbonate, as described in U.S. Pat. No. 5,000,787, expressly incorporated herein by reference. It has also been postulated that inks are more likely to produce misting when the ink is less extendible or elastic.
Applicants have discovered that printing inks employing the novel vinyl resins of the invention have surprisingly high resistance to misting and slinging on high speed presses.
A further problem that has been encountered for inks that include vinyl copolymer vehicles has been objectionable odor due to residual monomer. Ripley et al., in U.S. Pat. No. 4,327,011 (expressly incorporated herein by reference) disclose a styrene-acrylic copolymer in a low solvency hydrocarbon for a lithographic printing ink. The copolymer is a linear polymer having a weight average molecular weight of up to about 50,000 in an essentially aliphatic hydrocarbon solvent that boils in the range of 390-595xc2x0 F. While the authors report xe2x80x9cessentially complete conversion of the monomers,xe2x80x9d it has been found that copolymers prepared according to the instructions of the Ripley patent typically had objectionable odor due to incomplete conversion of the monomers, as even low levels of residual monomers may cause odor problems. In addition, the polymers of the,Ripley patent exhibit high tacks and have unacceptable misting and slinging. While tack may be lowered with additional solvent, the additional solvent causes an unacceptable reduction in ink body that introduces other problems in printing the ink.
Applicants have discovered an improved process that may be used to obtain vinyl polymers that have substantially no residual monomer and, consequently, are free of odor associated with residual monomer.
The invention provides a printing ink composition that includes a branched vinyl resin. The term xe2x80x9cvinyl resinxe2x80x9d when used in conjunction with the present invention includes polymers prepared by chain reaction polymerization, or addition polymerization, through carbon-carbon double bonds, using vinyl monomers such as acrylic and methacrylic monomers, vinyl aromatic monomers including styrene, and monomers compatible with these. By xe2x80x9cbranched vinyl resinxe2x80x9d it is meant that, while the vinyl polymer is branched, it nonetheless remains usefully soluble. By xe2x80x9csolublexe2x80x9d it is meant that the polymer can be diluted with one or more solvents. (By contrast, polymers may be crosslinked into insoluble, three-dimensional network structures that are only be swelled by solvents.) The branched vinyl resins of the invention unexpectedly retain solubility in spite of significant branching.
The branched vinyl polymers of the invention preferably include at least about 0.008 equivalents, per 100 grams of monomer polymerized, of at least one monomer having at least two ethylenically unsaturated polymerizable bonds or at least about 0.004 equivalents per 100 grams of monomer polymerized of each of two ethylenically unsaturated polymerizable monomers having mutually reactive groups other than the polymerizable double bonds. The branched vinyl resin typically has a low number average molecular weight and a broad polydispersity. In a preferred embodiment, the branched vinyl resin of the invention has a polydispersity of at least about 15, as determined by gel permeation chromatography calibrated with polystyrene standards according to well-known methods.
The invention also provides a method of making an ink composition with the branched vinyl resin. In another aspect of the invention, the vinyl-based printing ink is modified by the addition of another vehicle resin. The invention further provides a method of polymerizing a branched vinyl resin in which substantially no monomer remains unpolymerized. The invention also provides a method of printing using the compositions of the invention.
Ink formulations including the branched vinyl vehicle of the invention have unexpectedly improved misting and slinging properties. The inks using the branched vinyl polymer have reduced tack at higher viscosities relative to comparable inks made with linear vinyl polymer vehicles. The invention reduces the amount of misting and slinging and improves the tack/body balance as compared to what would be expected for previous inks formulated with vinyl vehicles that would provide body by increasing tack or nonvolatile content or both. Incorporation of the branched vinyl vehicles of the invention into ink compositions also lends a certain amount of elastic character to the inks, which in many cases is beneficial to ink properties. News inks that include the branched vinyl polymer of the invention have improved rub-off properties.
Finally, the inks of the invention containing the branched vinyl can be formulated as single-fluid printing inks. The single-fluid printing ink of the invention is advantageously employed in lithographic printing processes.